Novel domain-targeted anti-Trop-2 monoclonal antibodies to elicit therapeutic synergy against multiple human cancers.

Background: Trop-2 is a calcium signal transducer that drives tumor cell growth. Trop-2 is overexpressed in the majority of carcinomas, where it associates with worse prognosis. The Trop-2 extracellular domain contains a N-terminal cysteine-rich globular domain followed by a stem-like cysteine-less region that connects to the transmembrane domain. Trop-2 engages in homophylic interactions between adjacent cells and interacts with tight-junction proteins, which may severely affect accessibility by therapeutic monoclonal antibodies (mAb). Available anti-Trop-2 mAb mostly recognize a single immunodominant epitope between the globular and stem regions, and have limited or no therapeutic efficacy. In order to untap the potential of anti-Trop-2 immunotherapy we generated novel anti-Trop-2 mAb with tailored specificity towards the globular versus stem regions. Methods: Balb-C mice were immunized with soluble human Trop-2 produced in human 293 and murine L cell lines and in baculovirus expression system. Domain-targeted anti-Trop-2 mAb were selected by flow cytometry using live 293 transfectants.Therapeutic potential was assessed in human cancer xenografts in murine models. mAb mode of action was investigated by Western blot, live-cell imaging and in vitro ADCC and cancer cell growth inhibition assays. Results: mAb were identified that were differentially directed against the Trop-2 stem versus globular regions. These mAb efficiently bound Trop-2 expressing cancer cells and inhibited cell growth in vitro. In vivo, naked anti-globular OX-G64 and anti-stem OX-S55 mAb were most effective in inhibiting the growth of colon, ovary, breast and prostate cancers as xenografts in nude mice. NSG mice and in vitro mechanism profiling indicated efficient ADCC, together with efficient internalization for ADC development. Differential efficacy for established tumors versus isolated-cell models of metastatic dissemination was shown. Most remarkably, OX-G64/OX-S55 co-administration demonstrated sinergic growth inhibition in vivo. Conclusions: The OX-G64 and OX-S55 anti-Trop-2 mAb are novel domain-targeted, sinergic therapeutic mAb for game-changing anti-cancer immunotherapy.